The invention relates to a squeegee device intended for use in a screen-printing device. By way of example, in a rotary screen-printing device, printing medium is pressed onto a substrate through a cylindrical stencil by means of a squeegee element. One type of squeegee element which is frequently used, in particular in textile printing, is the squeegee roller. In this case, the squeegee element is formed by a solid or hollow metal roller which, in an operating position, rests in the stencil at the location of the printing point. During the printing process, the stencil bears against the substrate which, in turn, is supported by substrate-conveyor means, for example a printing belt. Beneath the printing belt there is a magnetic beam, by means of which the squeegee roller is attracted firmly onto the stencil and the substrate. The squeegee roller is arranged in the stencil in such a way that it can move between the said operating position and an at-rest position, in which it is clear of the stencil. The at-rest position is desired, for example, during temporary interruption of a printing process during which the stencil is lifted off the substrate and continues to rotate while the substrate is at a standstill. The aim of this is to prevent leakage of printing medium out of the stencil and to prevent printing medium from drying inside the stencil. To prevent printing medium from being pressed out of the stencil in this situation, and to prevent excessive loads being imposed on the stencil, the squeegee roller should at this time not rest upon the stencil.
By way of example, EP-A-0,408,704 has disclosed a squeegee device with an elongate support frame which can be accommodated in a screen-printing device suspension means located outside the stencil. On the support frame, there are a plurality of permanent magnets which are able to attract the squeegee roller and hold it in place when a magnetic field beneath the printing belt is removed. The squeegee roller then moves into the at-rest position, bearing against a designated wall part of the support frame. If the magnetic field beneath the printing belt is reapplied, this field has to be powerful enough to pull the squeegee roller off the permanent magnets in the support frame.
A drawback of this known squeegee device is that the permanent magnets in the support frame, during the printing process, exert a constant, considerable force on the squeegee roller, counteracting the attracting force of the magnetic beam beneath the printing belt. This translates into high demands being imposed on the strength of the squeegee device and leads to considerable difficulties in dimensioning the various magnetic fields with respect to one another. In an attempt to avoid these difficulties, in the state of the art squeegee device, according to EP-A-0 408,704 the permanent magnets are positioned in the support frame at a vertical distance above the bottom end of the support frame. However, this in turn has the drawback of attenuating the magnetic field at the location of the operating position. This is because, on the one hand the permanent magnets in the support frame have to be strong enough to lift the squeegee roller out of its operating position, and on the other hand the permanent magnets must not be too strong, in order to ensure that the magnetic beam beneath the printing belt is able to pull the squeegee roller out of its at-rest position on the support frame. It has been found that, in practice, it is not easily possible to satisfy both requirements at the same time in the squeegee device according to the state of the art. During the printing process, a considerable counteracting force is exerted constantly on the squeegee roller. As a result, it is difficult to allow the squeegee roller to run sufficiently clear from the support frame. Actually, the squeegee roller in the state of the art squeegee device maintains positive contact with the support frame at all times. As a result, the squeegee roller is braked, which results in excessive wear of support frame and squeegee roller, and imposes loads on the stencil. In order to avoid such disadvantageous contact, a gap must be set between the squeegee roller and the support frame during printing in the operating position. The gap width between the squeegee roller and the support frame will vary over the longitudinal direction of the squeegee device owing to deformations of the support frame caused by compensating counteracting forces. The gap width variation and the necessity to prevent contact between the squeegee roller and the support frame over the whole length of the squeegee, results in a gap width setting having local trajectories along part of the squeegee length with enlarged gap widths, with the risk of printing medium flowing onto the wrong side of the squeegee roller. A further drawback is that the magnetic beam beneath the printing belt can only pull the squeegee roller out of its at-rest position at relatively short distances. Consequently, outside the sphere of influence of the magnetic beam, the squeegee roller is always in the at-rest position.
The object of the invention is to overcome the abovementioned drawbacks and, in particular, to provide a squeegee device which operates successfully and can also be used on existing screen-printing devices.
According to the invention, this object is achieved by means of a squeegee device according to claim 1. The squeegee device interacts with a squeegee element, for example a squeegee roller or squeegee blade, at least part of which is made from a magnetizable material. The squeegee element can move between an operating position and an at-rest position and is limited by a support frame. On the support frame, there are magnetic means for generating an attractive force from a magnetic field at the location of a bearing-wall part of the support frame. Furthermore there are switching means which interact with the magnetic means. In the switched-on position, the magnetic field is applied at the location of the bearing-wall part, while in the switched-off position the magnetic field is removed at the location of the bearing-wall part. The magnetic field is sufficiently strong to pull the squeegee element out of the operating position into the at-rest position against the bearing-wall part in the switched-on position. Advantageously, during a printing process the squeegee element is virtually only subject to a magnetic field which is generated beneath substrate-conveyor means, and the magnetic field from the support frame can then be switched off or removed. When the printing process is stopped, the magnetic field from the support frame can be switched on or applied, and the magnetic field beneath the substrate-conveyor means can be switched off or removed. As a result, counteraction of the magnetic forces is eliminated, and it is possible to impose less strict demands on the strength of the squeegee device. This device can be of more lightweight design, which is important in particular for long squeegee devices (broad printing widths), and also considerably increases ease of handling. In the operating position, the gap width between the squeegee element and the support frame can be kept within accurately defined limits. Deformations to the squeegee device caused by the magnetic forces counteracting one another no longer arise, making it possible to achieve gap widths of approximately 3 mm without there being any risk of the squeegee element running in contact with the support frame during the printing process. In the switched-on position, the magnetic field from the magnetic means on the support frame can be arranged as far as possible towards the bottom of the support frame and can be designed less powerful, since the distance between the operating position and the at-rest position is short. As a result, the magnetic means can be of more lightweight design. Furthermore, this shorter distance allows even roller squeegees of small to very small diameter to be picked up out of the printing medium and moved into the at-rest position.
In a preferred embodiment the magnetic means comprise one or several permanent magnets positioned in a row, for example mounted on a longitudinal holder body. With this the switching means comprise movement means for moving the permanent magnets between the switched-on and the switched-off position, for example towards and away from the designated bearing wall part against which the squeegee-element comes to lie in the at-rest position. The movement means may be constructed in a number of ways. In an advantageous embodiment the movement means comprise spring means for moving the permanent magnets into their switched-on position, and hydraulic or pneumatic means for moving the permanent magnets back into their switched-off position. As long as there is no pressure on the hydraulic or pneumatic means, the permanent magnets are being pushed by the spring means into the switched-on position. In this position the maximum magnetic force is present for holding the squeegee-element in its at-rest position. If subsequently pressure is built-up in the hydraulic or pneumatic pressure means, the magnets are forced backwardly to their switched-off position, and the squeegee element is given the freedom to move towards its operating position. This embodiment has the major advantage of being reliable, and easy to manufacture at low costs. In the case of a malfunction in the hydraulic or pneumatic pressure means, the squeegee-element is automatically forced into its at-rest position. This has the advantage that at all times it is possible to dismount and remove the squeegee device together with its squeegee element. There is no dependency on external energy sources like pressure or electrical means.
Other preferred embodiments of the invention are defined in claims 4-18.
The invention also relates to a screen-printing device according to claims 19 and 20 and a method for washing at a printing station of a screen-printing device according to claims 21 and 22 and to a squeegee with squeegee device and squeegee element according to claim 23.